By Patricia Lunn Adsit, Master Gardener Extension Volunteer with Walton County
It’s that time of year that makes my heart sing: my roses are beginning to bloom! I just love to walk in the garden to see which ones have opened overnight, always eager to fill a vase with as many scents as possible.
Unfortunately for us rose growers, it is also the time of year we have come to fear. When the new growth begins in earnest, the signs of a deadly enemy become evident: a nasty, viral rose-killing disease called Rose Rosette (RRD), which is spread by a microscopic species of a sausage-shaped Eriophyid mite whose tainted saliva trickles into the tissue, injecting the plant with the always-deadly plant virus. The mite, Phyllocoptes fructiphylus, is active from May through October, and though wingless, is easily carried on air currents. The saddest part is that there is currently no cure for diseased plants, which should be bagged on site to keep from dislodging any mites onto neighboring roses; then the roots should be dug up, and the entire plant disposed of. DO NOT COMPOST! Remove it from your property.
How do you know if your rose has RRD? Disease symptoms can vary depending on the species and cultivar of rose. According to Alabama Extension Pathologist Jim Jacobi, “the most common symptomsinclude: ‘witch’s brooms’ or abnormal clustering of small branches, distorted leaf growth, reddening of leaves, rapid stem elongation, stems that are thicker than parent stems, and excessive thorn production, with the thorns being very pliable, and red or green in color.”
This disease has been reported since the early 1940s, when the eriophyid mite, was known to be the transmitting agent, but the disease-causing agent was still a mystery. In 2011, research demonstrated that RRD is caused by a virus, fittingly named the Rose Rosette Virus (RRV). Coincidentally, that is the same year that I trained as a Master Gardener and found a RRD-infected red Knock-out TM rose in the Extension’s Demonstration Garden, earning the dubious honor of being the first to document an incidence of RRD in the Piedmont.
Research has discovered that a species named Rosa multiflora, which was extensively planted in the past century as hedgerows but which is now considered an invasive weed, caused RRD to spread from West to East across much of the United States, carrying the mite and the virus with it. Then we compounded the problem by massively planting landscape roses, particularly those red Knock-OutsTM, which may have a reputation of shrugging off black spot, but which are especially prone to RRD. It was as if we were creating an all-you-can-eat buffet for the mites!
So is there any good news about RRD? Actually, there is. Where we were previously advised not to replant another rose in a spot where a RRD-infected bush had been removed, new research has shown that the causal agent of RRD is not soil- borne. Therefore, it is possible to successfully plant healthy, new roses where diseased plants were without a waiting period, as long as all pieces of roots have been removed. Additionally, thanks to a five-year, four-state trial of some 600 rose types, approximately 30 roses with some promise of RRD-resistance have been identified. While there are no roses that are immune to RRD and none that can yet be classified as “RRD-resistant” (although there are some breeders who are making that claim prematurely), we are getting closer to that point.
Meanwhile, try to avoid the disease by purchasing clean stock from reputable sources, maintaining good spacing between rose bushes, and keeping your roses in good shape with regular pruning and proper fertilization. Is there is a chemical control for the virus that causes RRD? Sadly, not as of this writing, but miticides registered for control of the eriophyid mites may reduce the risk of spread of the disease, although you run the risk that chemicals may also kill beneficial predatory mites. Personally, I tend to rely on horticultural oils, insecticidal soap, and neem oil. One interesting cultural control method I intend to incorporate in my new rose garden is the use of non-host barriers, such as Miscanthus sinensis, which have been shown to reduce the appearance of symptoms by more than 50%, apparently by blocking the spread of the wind-borne mite.
Want to learn more about what to do in your garden this Spring? The Walton County Master Gardeners will be offering a series of talks at the O’Kelly Memorial Library, 363 Conyers Rd in Loganville 30052 (reservations 770- 466-2895) from 2:00-3:00 on Wednesdays and at the Monroe-Walton County Library, 217 W. Spring St, Monroe 30655 (reservations 770-267-4630) from 10:30-11:30 on Saturdays in May.
Have home gardening questions? Need a soil test kit? The Master Gardener Help Desk is staffed on Mondays (9:00-12:00) and Wednesdays (1:00-4:00). You can also email us at firstname.lastname@example.org or call 770-267-1925. Look for our Ask a Master Gardener booth on Saturday mornings at the Farmers Market in Monroe, May-October.
Patricia Lunn Adsit is a Walton County Extension Master Gardener Volunteer as well as a member of the Garden Writers Association (GWA). She lives and gardens in Loganville, GA